1. Field of the Invention
This invention relates to a projection display device and, more particularly, to a three-tube projection display device for projecting color television pictures on a screen with projection lenses by converging three primary color images displayed on three color projection cathode-ray tubes of red, green and blue.
2. Description of the Prior Art
An optical system for use with a prior-art projection television display device includes three color projection lenses, two of which are inclined relative to a screen (FIG. 1). In such prior-art projection television display device, a green projection tube 10G is normally placed at the center and a red projection tube 10R and blue projection tube 10B are placed at respective opposite sides thereof. Projection lenses 20R, 20G and 20B are then arranged in front of the red projection tube 10R, green projection tube 10G and blue projection tube 10B, respectively.
To compensate image distortions by the projection lenses, optical couplers 30R, 30G and 30B are interposed between the projection lenses 10R, 10G and 10B and the respective three color projection tubes 10R, 10G and 10B.
Three primary color images displayed on the three color projection tubes 10R, 10G and 10B are converged and focused on a screen 40 by means of the projection lenses 20R, 20G and 20B for reconstituting color television pictures thereon.
In projecting the three primary color images on the screen, there is no problem for the projection lens 20G since an optical axis 20G.sub.s thereof is arranged normal to the screen 40; however, there are some problems for the optical lenses 20R and 20B since optical axes 20R.sub.s and 20B.sub.s of the projection lenses 20R and 20B at the right and left sides of the projection lens 20G are inclined relative to the screen 40.
Accordingly, in such a projection display device as described above, face plates 10R.sub.a and 10B.sub.a of the projection tubes 10R and 10B are inclined, in accordance with what is called the Scheimpflug rule, by an amount of .beta. relative to the respective lens surfaces of the projection lenses 20R and 20B to provide entirely focused pictures on the screen 40, even though the optical axes of the projection lenses are inclined thereto (FIG. 2).
The inclined angle of .beta. (hereinafter referred to as a "Scheimpflug angle") between the lens surface and the face plate of the projection lens has the following relation with the projection screen: EQU (1/m) tan .theta.=tan .beta.
where,
m=magnification (optical power) of the projection lens 20 PA1 .theta.=the angle between the lens surface of the projection lens 20 and the screen 40 PA1 R=curvature of plano-convex face plate (curved on the side of phosphor) of the projection tube PA1 m=magnification of the projection lens PA1 n=refractive index of the coolant PA1 .theta.=angle of incidence to the screen
In FIG. 3, there is shown an optical coupler 30 coupled with the face plate of the projection tube 10. The optical coupler 30 is provided with a meniscus lens 30b, which is for compensating distortion of the projected image on the screen, mounted on a center-holed optical coupler frame 30a on the side thereof facing the projection lens 20 by employing a packing 30c, and then attached to the face plate 10a of the projection tube 10 on the side thereof facing the projection tube 10 by employing another packing 30d.
A space formed by the meniscus lens 30b, an inner wall of the optical coupler frame 30a and the face plate 10a of the projection tube 10 is filled with a liquid coolant for cooling the face plate 10a. The optical axis of the meniscus lens 30b is aligned with that of the projection lens 20.
Further, in FIG. 3, the angle between the meniscus lens 30b and the face plate 10a of the projection tube 10 is set to the Scheimpflug angle of .beta.; however, it is possible to arrange the meniscus lens 30b in parallel with the face plate 10a of the projection tube 10 and incline, as shown in FIG. 4, the optical axis of the projection lens 20 by an angle of .beta. with respect to the optical axis of the projection tube 10 for providing the same effect as that of the configuration shown in FIG. 3. In FIGS. 3 and 4, element 20a designates a projection lens frame for mounting the projection lens 20 on the optical coupler frame 30a.
In the projection television display device of the type as described above, if the size (normally indicated by the diagonal length in inches) of the projection picture is changed, the angle .theta. between the optical axes of the projection lens at the center and each of the projection lenses at the right and left (or the angle .theta. between the projection lens surface and the screen) and the magnification m of each projection lens must be varied in accordance with the projection distance and, accordingly, the Scheimpflug angle .beta. will also be varied in accordance with the size of projection picture.
In accordance with the prior art projection television display device, therefore, there have been employed a number of dedicated optical coupler frames in order to provide different Scheimpflug angles .beta., between the surface on which the meniscus lens is mounted and the surface which the face plate abuts, in accordance with the picture size selected to be projected on the screen. However, since the optical coupler frame of this kind has normally been manufactured by aluminum die casting and the like, a number of molds have been prepared for different projection picture sizes, thus resulting in a problem of high production cost.
Further, it has been required to incline the projection tubes precisely in order to provide the registration of three primary color images on the screen. This in turn has caused problems such that tight toreance mounting parts are required to mount the projection tubes whenever the projection picture size is changed and, moreover, no manufacturing jigs can be used in common for producing such mounting parts.
It is therefore an object of this invention is to eliminate problems encountered by the prior art projection television display device and to provide a projection television display device wherein an optical coupler frame can be utilized for any picture size of projection television display by off-setting the centers of a meniscus lens and projection lens from that of a projection tube.